on 16/06/2019, Michael Chare supposed :
I have a charger for a Dyson V59. The nominal output voltage is 26v. My meter
shows the actual output voltage when not charging is 23.1v

Checking the voltage without a suitable load, is absolutely
meaningless.

Many chargers don't output a voltage to a battery, unless they 'see'
the voltage of a battery at the output.